The present invention generally relates to a semiconductor device, and more particularly relates to a semiconductor device including a fluorine-containing insulating film, made of an insulator doped with fluorine, between metal interconnects and a method for fabricating such a device.
Hereinafter, a semiconductor device including a fluorine-containing insulating film, made of an insulator doped with fluorine, between metal interconnects will be described with reference to FIG. 9.
As shown in FIG. 9, an insulating film 2, made of silicon dioxide, is formed on a semiconductor substrate 1. on the insulating film 2, a plurality of lower-level metal interconnects 3 are formed. Each of these metal interconnects 3 has a multilayer structure formed by stacking a first titanium film 3a, a first aluminum alloy film 3b and a first titanium nitride film 3c in this order. A fluorine-containing silicon dioxide film 4 is formed by doping a silicon dioxide film with fluorine to fill in the gaps between adjacent ones of the lower-level metal interconnects 3 and to cover all of these metal interconnects 3.
An ordinary silicon dioxide film 5 is formed on the fluorine-containing silicon dioxide film 4. And on the silicon dioxide film 5, a plurality of upper-level metal interconnects 6 are formed. Each of these metal interconnects 6 also has a multilayer structure formed by stacking a second titanium film 6a, a second aluminum alloy film 6b and a second titanium nitride film 6c in this order.
The relative dielectric constant of the fluorine-containing silicon dioxide film 4, formed by doping a silicon dioxide film with fluorine, is lower than that of the ordinary silicon dioxide film 5, which is not doped with fluorine. Thus, by providing an interlevel insulating film including the fluorine-containing silicon dioxide film 4 to fill in the gaps between the lower-level metal interconnects 3 and cover all of these interconnects 3, parasitic capacitances between the interconnects 3 themselves and between the lower- and upper-level metal interconnects 3, 6 can be reduced. As a result, signals can be transmitted with a reduced delay, thus enabling the use of signals with a higher frequency.
However, if the fluorine-containing silicon dioxide film 4 is interposed between the lower- and upper-level metal interconnects 3, 6, then fluorine, contained in the fluorine-containing silicon dioxide film 4, is likely to diffuse into the silicon dioxide film 5 during a subsequent heat treatment process. Such a phenomenon is called xe2x80x9cauto-dopingxe2x80x9d in the pertinent art. Once fluorine has diffused into the silicon dioxide film 5, a degraded layer is formed in the interface between the second titanium film 6a, which is the lowermost layer of the upper-level metal interconnects 6, and the silicon dioxide film 5. Accordingly, the upper-level metal interconnects 6 come to make poorer contact with the silicon dioxide film 5, resulting in deterioration in reliability of the semiconductor device.
An object of this invention is improving the contact between upper-level metal interconnects and an interlevel insulating film by preventing fluorine, contained in a fluorine-containing silicon dioxide film filling in the gaps between lower-level metal interconnects and between the lower- and upper-level metal interconnects, from degrading the upper-level metal interconnects.
To achieve this object, the semiconductor device of the present invention includes: a plurality of lower-level metal interconnects formed over a semiconductor substrate; a first fluorine-containing insulating film made of a fluorine-doped insulator and formed to fill in gaps between adjacent ones of the lower-level metal interconnects over the semiconductor substrate; an interlevel insulating film formed over the lower-level metal interconnects and the first fluorine-containing insulating film; and a plurality of upper-level metal interconnects formed on the interlevel insulating film. The interlevel insulating film includes: a second fluorine-containing insulating film made of a fluorine-doped insulator; and a silicon-rich insulating film containing a larger quantity of silicon than a quantity defined by stoichiometry.
In the semiconductor device of the present invention, the first fluorine-containing insulating film is formed between adjacent ones of the lower-level metal interconnects land the second fluorine-containing insulating film is interposed between the lower- and upper-level metal interconnects. Accordingly, parasitic capacitances between the lower-level metal interconnects themselves and between the lower- and upper-level metal interconnects can be reduced, thus reducing the propagation delay of a signal transmitted through the device.
In addition, the interlevel insulating film includes a silicon-rich insulating film containing a larger quantity of silicon than a quantity defined by stoichiometry. Thus, during a subsequent heat treatment process, even if fluorine atoms, contained in the first or second fluorine-containing insulating film, are going to diffuse, these fluorine atoms are trapped by the dangling bonds of silicon atoms contained in the silicon-rich insulating film. As a result, the fluorine atoms are less likely to pass through the silicon-rich insulating film to reach the upper-level metal interconnects. Accordingly, no degraded layers are formed in the interface between the lower- or upper-level metal interconnects and the interlevel insulating film, thus improving the contact between the lower- or upper-level metal interconnects and the interlevel insulating film.
If a silicon-rich insulating film is formed over the second fluorine-containing insulating film, then the silicon-rich insulating film prevents the fluorine atoms, contained in the first and second fluorine-containing insulating films, from reaching the upper-level metal interconnects. And, if an additional silicon-rich insulating film is formed under the second fluorine-containing insulating film, then the silicon-rich insulating films prevent the fluorine atoms, contained in the first and second fluorine-containing insulating films, from reaching the upper- and lower-level metal interconnects, respectively.
Therefore, in the semiconductor device of the present invention, not only reduction in parasitic capacitances between the lower-level metal interconnects themselves and between the lower- and upper-level metal interconnects, but also improvement of contact between the lower- or upper-level metal interconnects and the interlevel insulating film are realized. As a result, a highly reliable semiconductor device can be provided.
In one embodiment of the present invention, the silicon-rich insulating film preferably includes: a first silicon-rich insulating film formed under the second fluorine-containing insulating film; and a second silicon-rich insulating film formed over the second fluorine-containing insulating film.
In such an embodiment, the fluorine atoms, contained in the first fluorine-containing insulating film, cannot reach the upper-level metal interconnects, while the fluorine atoms, contained in the second fluorine-containing insulating film, cannot reach the upper- or lower-level metal interconnects.
In another embodiment of the present invention, the silicon-rich insulating film is preferably a silicon-rich oxide film containing a larger quantity of silicon than a quantity defined by stoichiometry.
In this embodiment, the refractive index of the silicon-rich oxide film is preferably 1.48 or more. Then, the passage of fluorine atoms through the silicon-rich oxide film can be prevented with much more certainty.
In an alternate embodiment of the present invention, the silicon-rich insulating film is preferably a silicon-rich nitride film containing a larger quantity of silicon than a quantity defined by stoichiometry.
In this embodiment, the refractive index of the silicon-rich nitride film is preferably 2.05 or more. Then, the passage of fluorine atoms through the silicon-rich nitride film can be prevented with a lot more certainty.
In still another embodiment, the first and second fluorine-containing insulating films are preferably fluorine-containing silicon dioxide films formed by doping respective silicon dioxide films with fluorine.
In such an embodiment, the parasitic capacitances between the lower-level metal interconnects themselves and between the lower- and upper-level metal interconnects can be reduced with even more certainty.
A method for fabricating a semiconductor device according to the present invention includes the steps of: a) forming lower-level metal interconnects and a first fluorine-containing insulating film over a semiconductor substrate, the first fluorine-containing insulating film being made of a fluorine-doped insulator and interposed between adjacent ones of the lower-level metal interconnects; b) forming an interlevel insulating film to cover the lower-level metal interconnects and the first fluorine-containing insulating film; and c) forming upper-level metal interconnects on the interlevel insulating film. The step b) includes the step of: forming a second fluorine-containing insulating film made of a fluorine-doped insulator; and forming a silicon-rich insulating film containing a larger quantity of silicon than a quantity defined by stoichiometry.
In accordance with the method of the present invention, an interlevel insulating film, including the second fluorine-containing insulating film made of a fluorine-doped insulator and the silicon-rich insulating film containing a larger quantity of silicon than a quantity defined by stoichiometry, can be formed with more certainty.
In one embodiment of the present invention, the step b) preferably includes: forming a first silicon-rich insulating film over the lower-level metal interconnects; forming the second fluorine-containing insulating film over the first silicon-rich insulating film; and forming a second silicon-rich insulating film over the second fluorine-containing insulating film.
In another embodiment of the present invention, the silicon-rich insulating film is preferably a silicon-rich oxide film containing a larger quantity of silicon than a quantity defined by stoichiometry.
In an alternate embodiment of the present invention, the silicon-rich insulating film is preferably a silicon-rich nitride film containing a larger quantity of silicon than a quantity defined by stoichiometry.
In still another embodiment, the first and second fluorine-containing insulating films are preferably fluorine-containing silicon dioxide films formed by doping respective silicon dioxide films with fluorine.
In this embodiment, the first and second fluorine-containing silicon dioxide films are preferably deposited by a plasma CVD process at a temperature in the range from 415xc2x0 C. to 460xc2x0 C.
Then, the fluorine atoms, contained in the fluorine-containing silicon dioxide films, are even less likely to be separated from the silicon atoms. Accordingly, the diffusion of fluorine atoms can be prevented with much more certainty.